1. Field of the Invention
The present invention relates to a telephoto lens system, through which an object to be inspected (hereinafter, a test object) at a finite distance is photographed at a high definition, and subsequently the photographed test object is image-processed.
2. Description of the Prior Art
Most of conventional telephoto lens systems have been designed for photographing lens systems. Such a conventional telephoto lens system cannot be used for the purpose of inspecting a test object. The process of inspection is generally performed by photographing the test object at a high definition, and subsequently by an image-processing operation for the photographed test object.
For example, Japanese Unexamined Patent Publication No. Hei-8-201687 has disclosed a four-lens-group telephoto lens system constituted by four lens elements; however, in the telephoto lens system thereof, the correcting of chromatic aberration is insufficient. Accordingly, the telephoto lens system of this publication is not suitable for an image processing operation at a high definition.
Furthermore, Japanese Unexamined Patent Publication Nos. Hei-5-27163, Hei-7-301749 and Hei-8-234098 can be taken as other examples. However, the lens system of each publication cannot be used for an image-processing operation at a high definition. Here, the high definition means a resolving power of approximately more than 80 curved lines of a Nyquist frequency with respect to the entire image plane.
In regard to a light source for image processing, there are cases where only visible light is used, or where a mixture of visible light and near-infrared light is used. Here, it has been known that a focal point undesirably moves largely between the case where visible light is employed as the light source and the case where the mixture of visible light and near-infrared light is employed as the light source.